1. Field of the Invention
This invention relates to a magnetic recording medium and more particularly, it is concerned with a multi-layer magnetic recording medium with improved magnetic recording performances.
2. Description of the Prior Art
As a magnetic recording medium there have hitherto been used those having magnetic recording layers of coating type consisting mainly of ferromagnetic powders and binders. However, this magnetic recording medium has the disadvantage that the magnetic recording density cannot be increased because of having a large proportion of a non-magnetic portion in the magnetic recording layer.
In order to overcome this disadvantage, the inventors have made studies on a method of using a ferromagnetic metal thin film as a magnetic recording layer. In the case of forming a ferromagnetic metal thin film, plating methods such as electroplating, chemical plating and electroless plating and vapor deposition methods such as vacuum vapor deposition and sputtering are well known. The inventors have hitherto proposed the methods described in Japanese Patent Publication Nos. 5301/1966, 20386/1966, 23892/1968, 1126/1969, 6309/1969, 9490/1969, 15507/1969, 18038/1972 and 33161/1973 and Japanese Patent Application (OPI) Nos. 14326/1974, 45703/1974 and 57396/1974 as a method using electroplating or chemical plating, the methods described in Japanese Patent Application (OPI) Nos. 15999/1974, 75004/1975, 115507/1975 and 116330/1975 and Japanese Patent Application Nos. 126364/1974 and 125915/1975 as a method of using electroless plating and the method described in Japanese Patent Application (OPI) Nos. 75005/1975 and 123304/1975. Furthermore, the inventors have also proposed, as a method of forming a metal thin film more suitable for use as a magnetic recording layer, ionic plating methods and electric field vapor deposition methods, which correspond respectively to a modification of the vapor deposition method. That is to say, the methods described in Japanese Patent Application (OPI) Nos. 33806/1975, 115508/1975 and 119609/1975 have been proposed as the ionic plating method and the method described in Japanese Patent Application No. 61628/1975 has been proposed as the electric field vapor deposition method.
The magnetic recording media having the ferromagnetic metal thin films obtained by these methods as a magnetic recording layer are particularly suitable for short wavelength recording. However, these methods meet with the problem, on the other hand, that if a ferromagnetic metal thin film with a sufficient thickness to effect a long wavelength recording is provided, various hindrances such as deterioration of the magnetic properties, lowering of the adhesiveness to supports, lowering of the surface smoothness and increase of the production cost tend to occur.
As a magnetic recording medium having improved magnetic recording properties whereby the above described problem can be solved, there have been proposed magnetic recording media of multi-layer coating type having two or more magnetic recording layers, as disclosed in Japanese Patent Publication Nos. 2218/1962, 23678/1964, Japanese Patent Application (OPI) Nos. 31602/1972, 37903/1972, 31907/1973 and 31804/1975, U.S. Pat. Nos. 2,643,130, 2,647,954, 2,941,901 and 3,676,217 and DT-AS Nos. 1190985 and 1238072.
Of such multi-layer magnetic recording media, the following advantages are given in particular, where the uppermost layer is a ferromagnetic metal thin film.
(1) Since a ferromagnetic metal thin film is positioned near a head and the magnetic field produced by the head can thus be utilized effectively, a sufficient magnetic recording can be carried out with the commonly used recording current even if the coercive force is relatively increased and a short wavelength recording can favourably be carried out. In addition, the erasure by an erasing head can sufficiently be carried out. PA1 (2) Since a ferromagnetic metal thin film can be used with a small thickness due to its high maximum residual magnetic flux density, the spacing loss resulting from departure of a coating type magnetic recording layer from a head can be minimized and the properties of the coating type magnetic recording layer can be taken out well by the usual recording and reproducing method. PA1 (3) The level of a possible magnetic recording can be raised. In the recording of an audio signal, the maximum output level (MOL) is increased and in the recording of a video signal, the maximum output is increased. PA1 (4) The abrasion resistance to a head can be improved. PA1 (5) The demagnetization by magneto striction of a magnetic layer of binder type can be improved. PA1 (6) When using the multi-layer magnetic recording medium as a magnetic recording tape, the resistance to bending as well as the durability of tape playing can be increased and troubles such as by disordered winding can be decreased. PA1 (7) Adhesion of dusts and generation of noises due to static charge can be decreased. PA1 (8) Contamination of a head due to powder scraping can be decreased. PA1 (9) When using the multi-layer magnetic recording medium as a magnetic recording tape, a tape vibration and wow flutter due to tape elongation can be prevented. PA1 (10) Since a ferromagnetic metal thin film is capable of giving sufficient effects with a small thickness, it can be produced with a relatively low cost.
In order to obtain the above described advantages by providing a ferromagnetic metal thin film on a magnetic recording layer of coating type, the ferromagnetic metal thin film must have an excellent surface smoothness as well as an excellent adhesiveness to the lower layer. When an ordinary magnetic recording layer of coating type consisting of a ferromagnetic powder and binder is subjected to a surface activating treatment by etching to give an anchor effect and to improve the adhesiveness and then to an electroless plating of a ferromagnetic metal thin film, for example, a smooth surface cannot be obtained and the adhesiveness to a layer under the ferromagnetic metal thin layer is often insufficient. Even by a method wherein a ferromagnetic metal thin film is provided by vapor deposition, the adhesiveness of the ferromagnetic metal thin film to the lower layer is not so improved and the above described advantages cannot be obtained.